1. Field of the Invention
The invention relates to the field of mountings for instruments used to sense conditions in the fuel assemblies of nuclear reactors such as pressurized water reactors, wherein the instruments and/or tubes holding the instruments must extend upwardly from the fuel assemblies, where there is a strong axially upward flow of water heated by the fuel rods. An insert for the thimble tube receiving the instrument tube has opposed bow springs and dimples protruding radially inwardly, for steadying the top of the instrument tube.
2. Description of the Prior Art
Nuclear reactors of the type used to generate power by producing heat in fissile material generally have reactor cores comprising numerous fuel rods arranged in closely spaced arrays. The fuel rods are carried in supporting skeletons which can be handled as units. The skeletons have a top plate and a bottom plate known as nozzles, and a number of intermediate plates known as grids. The grids have openings through which the fuel rods extend and are held vertical and parallel to one another in a laterally spaced array, being relatively rigidly held in place by spring formations on the grids. Nuclear flux passing between the fuel rods heats the fuel rods, and water or other coolant is circulated over the fuel rods to extract heat for useful generation of power. Heating of the water along the surfaces of the fuel rods produces a convection current moving the water vertically upwardly. The water passing over the top of the fuel rods in a strong axial current.
The top and bottom nozzles and the intermediate grids of the fuel assemblies are fixed relative to one another by thimble tubes attached respectively to the grids and nozzles, forming a structural unit for supporting a number of the fuel rods, for example about 300. The thimble tubes include control rod guide thimbles for passage of control rods which when lowered absorb nuclear flux to reduce the overall rate of fission, or when raised allow the flux to pass between fuel rods to increase the rate of fission.
Preferably, one of the thimbles for each fuel assembly is reserved for instrumentation which senses the operation of the reactor in the area of the fuel assembly. The instrumentation is in turn carried in a sensor tube which can be lowered into one of the thimbles for monitoring, or raised from its thimble for service. The thimbles for the control rods and for the instrumentation are located among the fuel rods. Typically, the instrumentation tube thimble is located centrally in the array of fuel rods, and the control rod guide thimbles are interspersed among the fuel rods in a pattern.
In order to achieve efficient fuel burn rates and assure safe operation, continuous monitoring of each fuel assembly within a reactor core is important. Proper monitoring is accomplished through the use of instrumentation probes. The probe apparatus includes the sensor or instrumentation tube holding the sensors, i.e., a stationary housing which extends the length of the fuel assembly, and an electronic sensor arrangement. The sensor arrangement can have one or more sensors on preferably well-protected cables, lowered into the housing and electrically coupled to signal processing apparatus. Preferably, the sensor arrangement is carried in an instrumentation tube which is lowered into the sensor thimble tube. A number of instrumentation tubes can be inserted into selected thimble tubes, including the central tube and/or others of the thimble tubes which otherwise would hold a control rod. The instrumentation tube normally is arranged to be withdrawn. Electrical conductors for conveying the sensor signals from the sensors in the instrumentation tube extend upwardly within an extension of the instrumentation tube, to appropriate remote electronics operable to decode useful information from the sensor signals. Although the top nozzle of the fuel assembly provides some protection, there is an unshielded expansion gap over an axial distance above the top of the sensor thimble, where the instrumentation tube, or perhaps the external shielding of a cable or the like leading to the sensors in the thimble, protrudes above the sensor thimble tube.
During reactor operation, cooling water enters from the bottom of the reactor and flows upward, through the fuel assembly. Some of this water may flow along the annular space between the inner wall of the sensor thimble tube and the outer wall of the sensor probe housing (or instrumentation tube). A substantial axial flow of water exists around the fuel rods and the thimbles. The high velocity turbulent water flowing around the fuel rods thimbles, and in particular the axial gap at the top of the sensor thimble, subjects the top of the instrumentation tube to substantial lateral vibration, resulting in hydraulically induced fretting as the instrumentation tube (and/or any other structure provided to protect the instrumentation tube and its conductors at the top of the thimble), vibrates laterally and impacts against the inner walls of the instrumentation thimble.
Such a condition tends to shorten the service life of the instrumentation tube as well as the instrumentation thimble. The prior art solution to the problem has been to build these parts of materials which are sufficiently durable to withstand the vibrations which are produced by the unavoidable axial flow of cooling water.
The known instrumentation thimble comprises a stainless steel outer tube and a zirconium alloy inner tube coaxial with the outer tube. The zirconium and stainless steel tubes are axially locked relative to one another by forming a series of annular bulges in the two, e.g., by sliding the tubes over one another and rolling the inner tube using radially outward pressure to form annular interlocking ridges. This stainless steel reinforcement of the instrumentation thimble is provided at the top of the instrumentation thimble, extending downwardly in the direction of an uppermost one of the grids.
The instrument tube housing the sensors can also be made using stainless steel. Stainless steel provides sufficient strength and hardness to withstand the vibration-induced impacts which occur at the top of the instrumentation thimble. Unfortunately, stainless steel of an appropriate composition to withstand the necessary stress and impacts normally contains cobalt. The cobalt in the stainless steel is activated by irradiation in the nuclear flux of the reactor. When the instrument tube is removed from its thimble for service, such as testing and calibration, nuclear decay of the cobalt or its byproducts in the stainless steel produces unwanted radiation that can be dangerous.
It would be advantageous to provide some means to render the instrument tube and the thimble more durable for surviving impacts, without relying on a material which results in the tube or thimble becoming radioactive after use. It would also be advantageous to reduce the vibration occurring at the top of the fuel assembly due to the axial rush of water, even though it is necessary to reactor operation that this rush of water be facilitated.